The invention relates to a return-to-zero receiver.
Referring to FIG. 1, quite often digital circuits 10 and 12 need to communicate with each other. For example, the circuit 10 might send digital bits to the circuit 12 over a communication line 14. The circuits 10 and 12 might be directly connected together. However, to avoid ground loops and DC mismatches between the two circuits 10 and 12, DC blocking elements, such as transformers or capacitors, might be used to couple the communication line 14 to the circuits 10 and 12.
In this manner, a capacitor 16 may be used to prevent the DC bias voltages of the circuits 10 and 12 from reaching the communication line 14. However, in doing so, the capacitor 16 removes the average, or DC, component of the stream of bits that is furnished by the circuit 10. As a result, the circuit 12 may encounter difficulties in recovering bits from the communication line 14 as the logic one and logic zero threshold voltages may not be constant. To avoid these difficulties, the circuit 10 may encode the bits via a return-to-zero (RZ) encoding scheme so that the encoded bit stream furnished by the circuit 10 does not have a DC component.
Referring also to FIG. 2, when RZ encoding is used, each logic one bit 27 is encoded into a pair of pulses: a positive going RZ pulse 24 and a negative going RZ pulse 26. The two RZ pulses 24 and 26 are complementary so that each pair of pulses does not disturb the DC level of the encoded bit stream. As a result, constant threshold voltage levels are used to detect the positive 24 and negative 26 going RZ pulses and thus, decode the bits.
An RZ transmitter 20 (see FIG. 1) of the circuit 10 encodes the positive edge of the bit 27 into the positive going RZ pulse 24 and encodes the negative edge of the bit 27 into the negative going RZ pulse 26. An RZ receiver 22 of the circuit 12 receives and decodes the RZ pulses 24 and 26 to form the bit 27.
If RZ encoding is used for a set of communication lines, such as a bus, an RZ transmitter and receiver pair is fabricated for each line of the bus. As a result, the size of the transmitters/receivers and the power consumed by the transmitters/receivers may present difficulties.
Thus, there is a need for an RZ receiver that improves one or more of the size, response time and power consumption of such system.